Here's a photo of an impossible triangle. The object is of my own design and was realized via 3D printing by Shapeways. The material is stainless steel, with Bronze Plated finish.

Note: No post-processing was used to achieve the illusion.

Spoiler: If you are curious how the illusion has been realized, you can have a look at this animation: fav.me/d688ndv. Or check out the additional pictures of the model that are available at my Shapeways shop www.shapeways.com/model/114450….

Thanks, but the reason it looks realistic is that this happens to be a photo I have also made 3D renders of impossible objects, but this is an actual photo for a change. I did not use any post-processing though to achieve this illusion. What you see is the photo as I made it. The "impossible" object is one that I designed myself and have realized via 3D printing. Check out the link to the animation in the description to see what the object actually looks like.

It's indeed nice that the photo does not give (strong) clues on how the illusion has been realized. With the curvy and connected model used here, you basically get for free that shadows and reflections are not shown. The illusion requires a frontal view, and the model's natural position is with only one of the triangle's sides on a surface. This in contrast with the more common, straight and disconnected model, which has one side pointing upwards and two on the surface. In this case, for the illusion to work you need to point the camera downwards and will see any shadows and reflections cast by the model.